The present invention relates in general to the field of static discharge dissipation, and more particularly, to a structure and method for dissipating electrostatic discharges about the surface of a sensor.
Without limiting the scope of the invention, its background is described in connection with the protection of integrated circuit fingerprint sensors from objects that may cause electrostatic discharges at or about a sensor surface, as an example.
Heretofore, in this field, the detection of fingerprint patterns, composed of lines or ridges and valleys, has been useful for the identification of specific individuals based on the observation that each individual person has a unique fingerprint. Fingerprints, therefore, can be used not only to positively identify individuals, but to exclude individuals whose fingerprint profile does not match a pre-existing set of patterns.
Fingerprint sensing has evolved from optical and mechanical sensing technologies that acquire a fingerprint image. In those systems, generally, the mechanical and optical sensors obtain a fingerprint image using a scanner or a camera, process the acquired information into an analog or digital signal that may be analyzed, and provide an output based on the acquired signal. Unfortunately, the lighting and contrast conditions available at the time the image is acquired affects the analysis of the acquired data and consequently affects the sensor output. Furthermore, image capture systems are easily tricked using a false images. In addition, conventional optical sensors usually require bulky optics, making these types of sensors impractical for portable systems.
Another class of fingerprint sensors are capacitive sensors, such as that disclosed in U.S. Pat. No. 4,353,056 issued to Tsikos. The Tsikos patent demonstrates the use of a sensor that incorporates a sensing member that has a sensing surface for receiving a fingerprint. The sensing surface has a means for sensing the ridges and valleys of the skin of the finger under observation. The sensing member contains a multitude of capacitors that sense the patterns of the fingerprint when the finger is pressed against the sensing surface. The information obtained is transformed into an electric signal. The capacitors are insulated from the environment of use by a flexible membrane that conforms itself to the contour of the fingerprint. Unfortunately, the repeated cycles of flexing and compression of the flexible membrane can lead to device failure and the need to replace the membrane.
U.S. Pat. No. 4,385,831 issued to Ruell, et al., discloses a fingerprint sensor that provides an electrical output signal in response to the topography of the fingerprint. The sensor incorporates a contact body that is formed, at least in part, by a light transparent elastic material. The elastic contact material may be attached to a flat sensor plate that has a light receiving surface. The sensor also incorporates a light source and a photodetector to measure the valleys and ridges of the fingerprint. The elastic nature of the contact body causes cycles of compression and flexing that again lead to the deterioration of the contact point between the sensor and the finger.
It has also been found that the current methods and structures for protecting sensors from the environment of intended use fail to address the distinct environmental exposures to which the sensors are exposed, in particular, electrostatic build-up on, e.g., human skin or any other object that may come into close proximity with or contact the sensor. Sensor protection versus sensitivity must generally be carefully balanced to achieve both an acceptable signal-to-noise ratio and adequate protection. Generally, as sensor protection increases, sensor sensitivity decreases. In the case of electrical damage to sensor surface structures or the active circuits that form part of the sensor circuitry during use, present electrostatic discharge circuitry fails to protect the sensor circuitry during an electrostatic discharge.
As sensors, and users, may be exposed to a wide variety of environmental conditions that can cause a great increase in electrical potential in comparison to objects that are at a different potential or grounded, it has been found that sensors should be fitted with electrostatic discharge protection to be durable. For example, when the user approaches the sensor at a great voltage disparity, a sudden electrical discharge may cause operational failure of the sensor, such failure may be temporary or permanent.
Typical electrostatic discharge protection circuits for solid state arrays may be relatively poor, since in this type of circuit configuration it is usual to connect the cell""s buried and ungrounded capacitor plates to transistor gates and/or to connect the cell""s ungrounded and buried capacitor plates to system ground potential by way of reverse biased diodes. In this type of construction and arrangement, the electrostatic charge sometimes carried by a human body and its fingertip, which may be in the range of several kilo volts (kV) or more, may be sufficiently high to break through the solid state cell""s upper dielectric/passivation layer. If this breakthrough occurs, the potential is raised at ungrounded circuit nodes that are associated with the buried capacitor plates and may cause damage to the associated array cell. Damage to the data or the sensor must be avoided, while the sensitivity of the sensor is maintained at close to optimal levels.
Another significant problem of the current structures for the protection of fingerprint sensors is contamination from substances, such as oils and proteins, that are found on the surface of fingers. To remove these contaminants, organic or inorganic solvents or detergents are often used to clean the sensor surface. Therefore, the electrostatic discharge protection must be resistant to these often corrosive compounds.
Another area of concern is hygiene. Fingers, as well as the environment, tend to contain microbes that are removed from the sensor along with contaminants. To remove these microbes and contaminants and reduce the chance of contagion between users, antibacterial, antifungal and decontaminating agents are often used to clean the sensors. These decontaminating agents often include harsh abrasives, enzymes, organic or inorganic solvents or detergents. Furthermore, the sensors are often exposed to oxidating environments, UV rays, and the like during normal use. Therefore, any electrostatic discharge protection must be resistant to corrosive cleaning compounds and other environmental agents that may damage the sensor.
What is needed is a structure and method to protect sensors from electrostatic discharges, while at the same time maintaining the sensors ability to withstand mechanical stress. The structure must not only permit continued functioning of the sensor during normal use, but also withstand, among others, the extreme conditions of humidity, electricity, heat, light, etc., to which the sensor may be exposed. The sensor electrostatic discharge structure should also be resistant to chemical detergents and solvents, but still be compatible with the underlying components of the sensor.
The present invention provides a method of, and system for, providing electrostatic discharge capability for a sensor circuit, such as a fingerprint sensor. In a type of sensor array with which this invention finds utility, each sensing cell includes ungrounded metal capacitor plates covered by a dielectric. An ungrounded object, such as a fingertip that comes close to, or in contact with, the dielectric forms a third capacitor plate between the metal capacitor plates. An electrostatic charge sometimes carried by a human body may be sufficiently high to break through the dielectric layer covering the metal capacitor plates. The present invention improves the electrostatic discharge performance of such a sensor array.
The present invention is directed to a process and apparatus for dissipating electrostatic charges including, an insulating layer between and over first and second conductive plates, wherein the insulating layer isolates the conductive plates and protects the conductive plates from damage. The insulating layer will generally be a dielectric layer. A conductive electrostatic discharge path is disposed over at least a portion of the dielectric layer and disposed adjacent to or over at least a portion of the conductive plates.